The present invention relates to an electrophotographic member having high sensitivity and excellent dark decay characteristics and capable of maintaining surface potential and dark decay characteristics after repeated use, and a composition of coating solution for forming a photoconductive layer and a composition for forming a charge transport layer used in such an electrophotographic member.
The conventional electrophotographic members have been produced by vacuum depositing an approximately 50 .mu.m thick selenium (Se) film on an electroconductive substrate such as aluminum. Such Se type electrophotographic members, however, have the problem that their sensitivity is limited to light with a wavelength of up to around 500 nm. An electrophotographic member is known in which an approximately 50 .mu.m thick Se layer is formed on an electroconductive substrate and a selenium-tellurium (Se-Te) alloy layer is further formed thereon to a thickness of several .mu.m. In this device, the spectral sensitivity can be elevated to a long wave-length region as the Te content in said Se-Te alloy is increased, but on the other hand, increase of the Te content deteriorates the surface potential retainability of the device, making it practically unusable for the intended purpose.
There is also known a laminate type electrophotographic member in which chlorocyan blue or a squarylium dyes derivative is coated to a thickness of about 1 .mu.m on an aluminum substrate to form a charge generation layer, and a high-insulance mixture of polyvinyl carbazole or a pyrazoline derivative and a polycarbonate resin is further coated thereonto a thickness of 10-20 .mu.m to form a charge transport layer. This electrophotographic member, however, has no sensitivity to light with a wavelength of 700 nm or above.
Many reports have been made recently on the improved versions of this laminate type electrophotographic members, that is, the laminated electrophotographic members having sensitivity at around 800 nm in the semiconductor laser oscillation region. In many of these laminated electrophotographic members, a phthalocyanine pigment is used as charge generating material, and on this charge generation layer of about 0.5-1 .mu.m thickness, a high-insulance mixture of polyvinyl carbazole or a pyrazoline or hydrazone derivative and a polycabonate or polyester resin is coated to a thickness of 10-20 .mu.m to form a charge transport layer.
A laminate type electrophotographic member has a wide scope of selection for the material used for forming the photosensitive layer, and a high-performance electrophotographic member can be provided by combining the best suited materials for the specific electrophotographic properties such as charging, dark decay, sensitivity, residual potential, repetition characteristics, plate life, etc., so that this type of electrophotographic member is now gaining ground in the art.
However, this laminate type electrophotographic member still involves some problems relating to static durability and repetition characteristics although mechanical durability is excellent. Especially the problem is pointed out that in repeated use of the member, the surface potential may sharply drop, causing a corresponding increase of dark decay, during the period from charging to development.
In order to improve such repetition characteristics or durability, it has been tried to incorporate various types of additives such as antioxidant in the composition. Such incorporation of additives could indeed provide certain improvements, but on the other hand it could cause a reduction of sensitivity or deterioration of other properties. Thus, in the prior art, it has been hardly possible to obtain a satisfactory electrophotographic member.
There have been proposed many electrophotographic members made of organic and inorganic materials, and among them, the function separated type member, in which the charge generation layer and the charge transport layer are separated from each other, has been offered to practical use as photosensitive member for copying machines and laser beam printers.
As the material of the charge transport layer, poly-N-vinylcarbazole compounds, pyrazoline derivatives, oxazole derivatives, oxadiazole derivatives, hydrazone derivatives, styryl derivatives and benzidine derivatives are well known.
The charge generation material and the charge transport material usually have per se no film forming properties; they are dispersed or dissolved in a solvent together with a binder resin, and the dispersion or solution is coated on an electroconductive substrate and dried to form a film.
The uniform film forming properties of the charge transport layer are an important subject in the electrophotographic process where long life of the elements is strongly required recently. Such uniform film forming properties of the charge transport layer are highly dependent on the layer composition, binder resin and solvent used therefor, so that proper selection of these materials is of much account. Generally, various types of polycarbonate resins are used as binder resin for the charge transport layer, while mixed solvents using a halogenated solvent are generally employed as solvent. Control of temperature and humidity is also important for forming a uniform charge transfer layer.
With a surge of the global movement for environmental protection of the earth in recent years, request is rising for total elimination of flon which destroys the ozone layer in the atmosphere and stronger regulation on use of halogen type solvents which may contaminate underground water, but there has yet been found no charge transfer layer composition which can meet these requirements.